Hummus is a great filling for a sandwich or a pita!
Also burritos are a good idea. If you want something hearty especially, fill it up with beans and/or rice.
Both of those are super easy options for someone new to being vegan, and are made with ingredients you are probably already familiar with. You can buy store-bought hummus and beans from the store and then add your own veggies. Lots of stores now (like trader joes) have some pretty good already baked/flavored tofu that you can add for protein. Also there are some good lunch-meat alternatives out there as well, like Tofurkey.
We also have a massive thread about packed lunches that might inspire you.viewtopic.php?f=18&t=17444
My advice is make a good dinner and bring leftovers to work/school with you! That's what I do 90% of the time. I just grab my tupperware with leftovers and some fruit/snacks and I am out the door.